1. Field of the Invention
This invention relates to an optical shutter for shielding a part of light beam selectively, and more particularly to an annular reflection mirror which can selectively reflect a part of light beam in the direction different from a progressing direction of the light beam and a manufacturing method thereof. Also, the present invention is directed to a different-type optical pickup device which can accurately access optical discs different in the depth extending from the surface thereof to the information recording face thereof by employing the annular shutter mirror.
2. Description of the Prior Art
In the recent recording medium, there has been required a capability of recording a higher capacity of information in accordance with the change into a large-scale of information quantity. Accordingly, an optical disc showing a remarkable increase of an occupation rate in the recording medium market also has been being developed such that it is possible to record a high capacity of information. As a result, in the optical disc market, there has appeared a digital versatile or video disc, hereinafter referred simply to as DVD, a recording capacity of which is about 6 to 7 times greater than that of the existing compact disc (CD) and the rewritable compact disc (CD-R). This DVD is not only higher than the CD and CD-R in a recording density, that is, in a track density, but also is shorter than those in a distance extending from the disc surface to the information recording face. In real, the distance from the disc surface to the information recording face in the DVD is 0.6 mm while that in the CD and CD-R is 1.2 mm.
As described above, different-type optical discs which have a different configuration are now commercially available in the optical disc market. Accordingly, an optical pickup device for accessing the optical disc also is required to have a capability of accessing all the different-type optical discs, that is, CD, CD-R and DVD and the like.
In order to satisfy the above requirement, there has been developed a holographic optical pickup device which comprises an objective lens system forming dual focus. This holographic optical pickup device allows a light beam, which is incident from a laser diode 10 via a partial reflection glass 12 to an objective lens 16, to be diffracted by means of a hologram lens 18, as shown in FIG. 1. The first-order diffracted light beam B1 allowing the objective lens 16 to maintain a numerical aperture of 0.4 and the zero-order diffracted light beam B0 allowing the objective lens 16 to maintain a numerical aperture of 0.6 are generated by the hologram lens 18. For the purpose of this, the hologram lens 18 is formed such that it has a concave surface of annular ring shape, and is combined such that the center of it is identical to that of the objective lens 16. Accordingly, the zero-order diffracted light beam B0 from the hologram lens 18 is focused onto the information recording face DF of the DVD in a shape of spot by the objective lens 16 while the first-order diffracted light beam B1 from the hologram lens 18 is focused onto the information recording face DS of the CD or CD-R in a shape of spot by the objective lens 16. Further, a multiple-division photo detector 22 converts into an electrical signal the zero-order diffracted light beam B0 which is reflected by the information recording face DF of the DVD and then is incident to it by way of the objective lens 16, the hologram lens 18, the collimator lens 14, the partial reflection glass 12 and the sensor lens 20 in turn and the first-order diffracted light beam B1 which is reflected by the information recording face DS of the CD or CD-R and then is incident to it by way of the objective lens 16, the hologram lens 18, the collimator lens 14, the partial reflection glass 12 and the sensor lens 20 in turn. As described above, the holographic optical apparatus could access the different-type optical discs by simultaneously forming the focus of light beam onto each of the information recording faces of the DVD and the CD/CD-R with the aid of the objective lens system having the hologram lens.
However, in the holographic optical pickup device, when a wavelength of the light beam generated at the laser diode changes according to a change of the external circumstance such as a temperature and the like, the diffraction angle of the diffracted light beam generated from the hologram lens changes. This change of diffraction angle in the diffracted light beam is caused to change the numerical aperture of objective lens, thereby generating an aberration. This allows a focus of light beam to be not formed on the information recording face of the DVD and the information recording face of the CD or CD-R. As a result, the holographic optical pickup device could not access the optical discs accurately when the temperature and the external circumstance, etc. changed. Also, the holographic optical pickup device had to experience a loss of light quantity depending on a diffraction efficiency of the hologram lens because it utilized only any one side of the zero-order diffracted light beam and the first-order diffracted light beam separated by the hologram lens in accordance with a type of optical disc. Due to the loss of light quantity, the holographic optical pickup device not only made a difficulty to do a signal process in the post-stage thereof, upon reading out information from the optical disc, but also it made a difficulty to irradiate a high power of light beam over a critical value required by an optical recording apparatus onto the optical disc. Further, the objective lens used for the holographic optical pickup device had nothing but to have a heavy weight because it must be formed in such a manner to be integral to the hologram lens. Owing to this heavy weight of the objective lens, the holographic optical pickup device could not access the optical disc at a high velocity.
In order to overcome such problems of the holographic optical pickup device, there has been suggested an optical pickup device employing a liquid crystal shutter and an optical pickup employing an annular shield. However, the former liquid crystal shutter type has a drawback in that it is necessary to use expensive polarizer and liquid crystal plate additionally. On the other hand, the latter annular shield type has a drawback in that a configuration thereof requires a high accuracy to made a mass production difficult. The above problems in these liquid Crystal shutter type and annular shield type of optical pickup device will be explained in detail with reference to FIG. 2 to FIG. 5 below.
Referring to FIG. 2A and 2B, there is shown an liquid crystal shutter type of optical pickup device which comprises an objective lens 44 for converging a light beam incident from a light source 30 via a collimator lens 32, a beam splitter 34, a orthogonal reflection glass 36, a liquid crystal plate 38 and a polarizer 40 in turn to it onto the first or second information recording face 42A or 42B of an optical disc 42. The collimator lens 32 converts a divergent light beam progressing from the light source 30 toward the beam splitter 34 into a parallel light beam. The beam splitter 34 allows a part, i.e., 50% of the light beam from the collimator lens to go toward the orthogonal reflection glass 36. Also, the beam splitter 34 allows a part, i.e., 50% of the light beam from the orthogonal reflection glass 36 to go toward the collimator lens, and simultaneously allows the remaining 50% of light beam to be reflected toward a sensor lens 46. The orthogonal reflection glass 36 makes a normal or perpendicular reflection of a light beam from the beam splitter 34 toward the liquid crystal plate 38, and simultaneously makes a normal or perpendicular reflection of a light beam from the liquid crystal plate 38 toward the beam splitter 34. The first information recording face 42A of the optical disc 42 is an information recording face of DVD, which is positioned on a depth of 0.6 mm from the surface of optical disc; while the second information recording face 42B of the optical disc 42 is an information recording face is an information recording face of CD or CD-R, which is positioned on a depth of 1.2 mm from the surface of optical disc. The sensor lens 46 converges the parallel light beam being incident from the beam splitter 34 into a multi-division photo detector 48. Subsequently, the multi-division photo detector 48 converts a light beam, which is reflected by the first or second information recording face 42A or 42B and then is incident thereto via the objective lens 44, the polarizer 40, the liquid crystal plate 38, the normal reflection glass 36, the beam splitter 34 and the sensor lens 46 in turn, into an electrical signal. This electrical signal generated at the multi-division photo detector 48 includes a servo signal for controlling a movement of the objective lens 44 in the up, down, left and right directions and an information signal recorded on the optical disc 42.
Further, the liquid crystal plate 38 is divided into a polarizing area 38A and an optional polarizing area 38B. The polarizing area 38A includes a liquid crystal layer, as not shown, in which liquid crystal particles are arranged in a constant direction, for providing a 90.degree. rotation in a polarized direction of incident light beam. Otherwise, the optional polarizing area 38B selectively provides a 90.degree. rotation in a polarized direction of incident light beam in accordance with an applied state of electrical field. For the purpose of this, the optional polarizing area 38B includes a liquid crystal layer, as not shown, in which liquid crystal particles are arranged in a constant direction, and a transparent electrode plate positioned on both sides of the liquid crystal layer. Also, the optional polarizing area 38B is connected with both a selection switch 50 and a voltage source 52 to form a circular circuit loop. In the case where the optical disc 42 is CD or CD-R, the selection switch 50 turns on such that a light beam going through the optional polarizing area 38B maintains the original polarized direction. On the other hand, in the case where the optical disc 42 is DVD, the selection switch 50 turns off such that a polarized direction of a light beam going through the optional polarizing area 38B is rotated by 90.degree.. Subsequently, the polarizor 40 transmits a light beam having such a characteristic that a polarized direction of a light beam from the liquid crystal plate 38 is identical to its own polarizing direction while it shuts off a light beam having such a characteristic that that is different from its own polarizing direction. As a result, since a flux diameter of a light beam being incident to the objective lens 44 by means of the liquid crystal plate 38 and the polarizor 40 and a numerical aperture of the objective lens 44 is varied, the liquid crystal type of optical pickup device becomes capable of accessing all the DVD and CD/CD-R. The varying process of the flux diameter of light beam and the numerical aperture of objective lens will be described in more detail which reference to FIG. 3A and FIG. 3B below.
FIG. 3A shows a state of light beams going through the liquid crystal plate 38 and the polarizor 40 in the case where the voltage source 52 is not connected with the optional polarizing area 38B of the liquid crystal plate 38, that is to say, the selection switch 50 turns off. In this case, all of light beams going through the polarizing area 38A and the optional polarizing area 38B of the liquid crystal plate 38 are rotated by 90.degree.such that they become identical to a polarized direction of the polarizor 40. All of light beams having the same polarizing characteristic as the polarizing direction of the polarizor 40 go through the polarizor 40, thereby increasing a numerical aperture of the objective lens 44. Accordingly, light beams of a smaller spot are formed on the first information recording face 42A positioned in a shallow depth from the disc surface, that is, on an information recording face of DVD by means of the objective lens 44, so that it becomes possible for the optical pickup device of liquid crystal type to access the DVD accurately. On the other hand, since light beams of much larger spot are irradiated onto the second information recording face 42B, that is, onto the an information recording face of CD or CD-R, it becomes impossible for the optical pickup device of liquid crystal shutter type to access the CD or CD-R.
FIG. 3B shows a state of light beams going through the liquid crystal plate 38 and the polarizor 40 in the case where the voltage source 52 is connected with the optional polarizing area 38B of the liquid crystal plate 38, that is to say, the selection switch 50 turns on. In this case, light beams going through the polarizing area 38A of the liquid crystal plate 38 are rotated by 90.degree. such that they become identical to a polarizing direction of the polarizor 40; while light beams going through the optional polarizing area 38B of the liquid crystal plate 38 maintain the original polarized direction, thereby having a polarizing characteristic different from the polarizing direction of the polarizor 40. At this time, since only light beams from the polarizing area 38A of the liquid crystal plate 38 having the same polarizing characteristic as the polarizing direction of the polarizor 40 go through the polarizor 40, a numerical aperture of the objective lens 44 decreases. Accordingly, light beams of a smaller spot are formed on the second information recording face 42 positioned in a remote depth from the disc surface, that is, on an information recording face of CD or CD-R by means of the objective lens 44, so that it become possible to access the CD or CD-R accurately. On the other hand, since light beams of much larger spot are irradiated onto the first information recording face 42A, that is, onto the an information recording face of DVD, it becomes impossible for the optical pickup device of liquid crystal type to access the DVD. The optical pickup device of liquid crystal shutter type therefore can access all of the CD and DVD by controlling a size of the polarizing area 38A of the liquid crystal plate 38 and a numerical aperture of the objective lens 44 properly and by switching a voltage applied to the optional polarizing area 38B of the liquid crystal plate 38 in accordance with a kind of optical disc, that is, in accordance with whether an optical disc to be accessed is CD or DVD.
However, the liquid crystal shutter type of optical pickup device has drawbacks in that, since it is necessary to use expensive liquid crystal plate and polarizor additionally for controlling a flux diameter of light beam, it becomes to take a complicated configuration and also its manufacturing cost become expensive.
FIGS. 4A and 4B show an annular shield type of optical disc which can access all of the CD, CD-R and DVD with the aid of an objective lens 60 with a certain width of annular band or stripe 60A being defined. Referring to FIG. 4, the objective lens 60 converges a light beam, which is incident from a laser diode 62 via a collimator lens 64, a beam splitter 66 and a orthogonal reflection glass 68 in turn thereto, onto the first or second information recording face 70A or 70B of a optical disc 70. The first information recording face 70A is an information recording face of DVD positioned in a depth of 0.6 mm from the surface of the optical disc 70, and the second information recording face 70B is an information recording face of CD or CD-R positioned in a depth of 1.2 mm from the surface of the optical disc 70. a light beam irradiated onto the first or second information recording face 70A or 70B is reflected by the information recording face 70A or 70B, and then is incident to a multi-division photo detector 74 via the objective lens 60, the orthogonal reflection glass 68, the beam splitter 66 and a sensor lens 72 in turn. The multi-division photo detector 74 converts a light beam being incident from the sensor lens into an electrical signal to thereby generate a servo signal for indicating a state of up, down, left and right movement in the objective lens 60 and an information signal relative to information recorded on the optical disc 70.
Further, the objective lens 60 is an objective lens for DVD with a numerical aperture of 0.6, which converges such that a light beam from the orthogonal reflection glass 68 defines a smaller spot on the first information recording face 70A, that is, on the information recording face of DVD. Accordingly, in the first information recording face 70A, a light quantity is centered only on the spot having a small dimension of .phi.1 without a side strobe, as shown in FIG. 5A, so that it become possible to access the DVD accurately. Otherwise, in the second information recording face 70B of the optical disc 70, that is, in an information recording face of CD or CD-R, a light quantity is distributed from a central main strobe region having a .phi.2 dimension until peripheral side strobe regions .phi.s. light fluxes irradiated onto the side strobe region .phi.s make a spot of light beam larger and also function as a noise, so that it becomes impossible to access the CD or CD-R accurately.
Furthermore, an annular band 60A defined in the objective lens 60 removes the side strobe regions generated at the second information recording face 70B, that is, an information recording face of CD or CD-R, whereby accurately accessing the DVD as well as the CD and CD-R. By this annular band, a light beam spot with a small dimension of .phi.2 is defined in the second information recording face 70B. In order to remove the side strobe regions .phi.s in the second information recording face 70B, the annular band 60A is defined in a partial region of the objective lens 60 generating the side strobe regions .phi.s in the second information recording face 70B, that is, in a region having a width of W from a position spaced by L from the center thereof.
However, in the annular shield type of optical pickup device, since the objective lens must be moved to the left or right direction thereof at the time of a control of tracking, the center of the objective lens and the annular band does not become identical to a light axis. This results in the side strobes being generated on the information recording face of CD or CD-R in accordance with a left or right movement of the objective lens. Moreover, in order to install the annular band in a region of objective lens generating the side strobe region in the information recording face of CD or CD-R accurately, a high accuracy is required in a configuration thereof. This made a difficulty in providing a mass production of the annular shutter type of optical pickup device.